Optimized graphite electrode pin configuration

ABSTRACT

A pin for use in connecting graphite electrodes into a joint, where the pin has at least one male tang having a tang factor, defined as the ratio of male tang length to diameter of the electrode into which hit is to be threaded, of at least about 0.60.

RELATED APPLICATION

This application is a continuation-in-part of copending and commonlyassigned U.S. patent application Ser. No. 10/830,618, filed Apr. 23,2004, entitled “Male-Female Electrode Joint,” the disclosure of which isincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to the configuration of pins used to joingraphite electrodes into an electrode column. More particularly, theinvention concerns a unique design for a pin, and the electrodes forwhich the pin is used.

2. Background Art

Graphite electrodes are used in the steel industry to melt the metalsand other ingredients used to form steel in electrothermal furnaces. Theheat needed to melt metals is generated by passing current through oneor a plurality of electrodes, usually three, and forming an arc betweenthe electrodes and the metal. Electrical currents in excess of 100,000amperes are often used. The resulting high temperature melts the metalsand other ingredients. Generally, the electrodes used in steel furnaceseach consist of electrode columns, that is, a series of individualelectrodes joined to form a single column. In this way, as electrodesare depleted during the thermal process, replacement electrodes can bejoined to the column to maintain the length of the column extending intothe furnace.

Conventionally, electrodes are joined into columns via a pin (sometimesreferred to as a nipple) that functions to join the ends of adjoiningelectrodes. Typically, the pin takes the form of opposed male threadedsections or tangs, with at least one end of the electrodes comprisingfemale threaded sections capable of mating with the male threadedsection of the pin. Thus, when each of the opposing male threadedsections of a pin are threaded into female threaded sections in the endsof two electrodes, those electrodes become joined into an electrodecolumn. Commonly, the joined ends of the adjoining electrodes, and thepin therebetween, are referred to in the art as a joint.

Given the extreme thermal stress that the electrode and the joint (andindeed the electrode column as a whole) undergoes, mechanical/thermalfactors such as strength, thermal expansion, and crack resistance mustbe carefully balanced to avoid damage or destruction of the electrodecolumn or individual electrodes. For instance, longitudinal (i.e., alongthe length of the electrode/electrode column) thermal expansion of theelectrodes, especially at a rate different than that of the pin, canforce the joint apart, reducing effectiveness of the electrode column inconducting the electrical current. A certain amount of transverse (i.e.,across the diameter of the electrode/electrode column) thermal expansionof the pin in excess of that of the electrode may be desirable to form afirm connection between pin and electrode; however, if the transversethermal expansion of the pin greatly exceeds that of the electrode,damage to the electrode or separation of the joint may result. Again,this can result in reduced effectiveness of the electrode column, oreven destruction of the column if the damage is so severe that theelectrode column fails at the joint section. Thus, control of thethermal expansion of an electrode, in both the longitudinal andtransverse directions, is of paramount importance.

Of course, the optimal way of achieving thermal compatibility betweenpin and electrodes is to form the pin of the same material from whichthe electrodes are formed; however, in conventional pin joints, the pinmust be formed of a graphite material that is stronger than the materialfrom which the electrodes are formed needs to be. If the pin is notformed of a stronger material, it would fail (i.e., suffer cracks andbreakage) to an unacceptable degree while in use in an electrode column.In order to avoid this, the pin can be formed, e.g., of a graphitematerial of a higher density than that needed for the electrodes.

What is desired, therefore, is a pin/electrode joint having sufficientstrength and integrity to reduce column damage in use and create a morestable joint, without a significant reduction in electrode performance.It is also highly desirable to achieve these property benefits withoutusing high quantities of expensive materials and, most advantageously,by use of the same graphite materials to form the pin as are used toform the electrodes in order to more closely match the thermalproperties of the pin and electrodes.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an improved pinconfiguration for graphite electrodes.

It is another object of the present invention to produce an improvedgraphite electrode joint using the inventive pin.

It is still another object of the present invention to provide a jointfor graphite electrodes which is designed to better withstand thethermal and mechanical stress on an electrode column in use.

It is yet another object of the present invention to provide a pin forgraphite electrodes which produces electrode column joints havingimproved strength and stability.

Another object of the present invention is to provide a pin for graphiteelectrodes formed of the same graphite materials used to form theelectrodes, without significantly affecting performance.

Still another object of the present invention is a graphite electrodejoint, having improved resistance to stub loss, defined as the loss ofthe part of the electrode column lying from the arc tip to and sometimesincluding the joint closest to the arc tip, as compared toart-conventional graphite electrode joints.

These objects and others that will become apparent to the artisan uponreview of the following description can be accomplished by providing apin for use in a forming a graphite electrode joint, the pin having atleast one male tang having a ratio of male tang length to diameter ofthe electrode on which it is to be used of at least about 0.60. In thepreferred embodiment of the application, the ratio of the diameter ofthe male tang to the length of the male tang should be no more thanabout 2.5 times the ratio of the length of the male tang to the diameterof the electrode on which it is to be used, when the ratio of the lengthof the male tang to the electrode diameter is about 0.60. Indeed, theratio of the diameter of the male tang at its base to the male tanglength should vary with the ratio of male tang length to electrodediameter such that for every 0.01 higher than 0.60 the ratio of maletang length to electrode diameter is, the ratio of the diameter of themale tang at its base to the male tang length should be about 0.016lower.

The inventive pin, when having a ratio of male tang length to electrodediameter of 0.85 or lower, should preferably also have a ratio of thetaper of the male tang, expressed in degrees, to the ratio of male tanglength to electrode diameter of at least about 15. Moreover, the ratioof the taper of the male tang to the ratio of male tang length toelectrode diameter varies with the ratio of male tang length toelectrode diameter such that for every 0.01 lower than 0.85 the ratio ofmale tang length to electrode diameter is, the ratio of the taper of themale tang to the ratio of male tang length to electrode diameter shouldbe about 1.25 higher.

The invention also includes an electrode joint formed from the inventivepin and two graphite electrodes, each having a female threaded socket,wherein a male threaded tang of the pins engages a female threadedsocket of each electrode to form the joint.

A process for preparing the inventive pin is also presented, includingmixing coke and a pitch binder, to form a stock blend; extruding thestock blend to form a green stock; baking the green stock to form acarbonized stock; graphitizing the carbonized stock by maintaining thecarbonized stock at a temperature of at least about 2500° C. to form agraphitized stock; and machining the graphitized stock so as to form amale tang having a ratio of male tang length to diameter of thegraphitized stock of at least about 0.60.

It is to be understood that both the foregoing general description andthe following detailed description provide embodiments of the inventionand are intended to provide an overview or framework of understanding tonature and character of the invention as it is claimed. The accompanyingdrawings are included to provide a further understanding of theinvention and are incorporated in and constitute a part of thespecification. The drawings illustrate various embodiments of theinvention and together with the description serve to describe theprinciples and operations of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side plan view of a pin in accordance with the presentinvention.

FIG. 2 is a partial side broken-away view of a graphite electrode jointemploying the pin of FIG. 1.

FIG. 3 is a partial side cross-sectional view of opposing graphiteelectrodes, each having a female socket for the pin of FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The pins used to connect graphite electrodes can be fabricated by firstcombining a particulate fraction comprising calcined coke, pitch and,optionally, mesophase pitch or PAN-based carbon fibers into a stockblend. More specifically, crushed, sized and milled calcined petroleumcoke is mixed with a coal-tar pitch binder to form the blend. Theparticle size of the calcined coke is selected according to the end useof the article, and is within the skill in the art. Generally, particlesup to about 25 millimeters (mm) in average diameter are employed in theblend. The particulate fraction preferable includes a small particlesize filler comprising coke powder. Other additives that may beincorporated into the small particle size filler include iron oxides toinhibit puffing (caused by release of sulfur from its bond with carboninside the coke particles), coke powder and oils or other lubricants tofacilitate extrusion of the blend.

Most preferably, the carbon fibers (when used) are preferably present ata level of about 0.5 to about 6 parts by weight of carbon fibers per 100parts by weight of calcined coke, or at about 0.4% to about 5.5% byweight of the total mix components (excluding binder). The preferredfibers have an average diameter of about 6 to about 15 microns, and alength of preferably about 4 mm to about 25 mm, and most preferably lessthan about 32 mm. The carbon fibers used in the inventive process shouldpreferably have a tensile strength of at least about 150,000 psi. Mostadvantageously, the carbon fibers are added to the stock blend asbundles, each bundle containing from about 2000 to about 20,000 fibers.

Preferably, the fibers are added after mixing of the particulatefraction and pitch has already begun. Indeed, in a more preferredembodiment, the fibers are added after at least about half the mix cyclehas been completed, most preferably after at least about three-quartersof the mix cycle has been completed. For instance, if the mixing of theparticulate fraction and pitch takes two hours (i.e., a mix cycle is twohours), the fibers should be added after one hour, or even ninetyminutes, of mixing. Adding the fibers after the mixing has begun willhelp preserve fiber length (which can be reduced during the mixingprocess) and thereby the beneficial effects of the inclusion of fibers,which are believed to be directly related to fiber length.

As noted above, the particulate fraction can include small particle sizefiller (small is used herein as compared to the particle size of thecalcined coke, which generally has a diameter such that a major fractionof it passes through a 25 mm mesh screen but not a 0.25 mm mesh screen,and as compared to the fillers conventionally employed). Morespecifically, the small particle size filler comprises at least about75% coke powder, by which is meant coke having a diameter such that atleast about 70% and more advantageously up to about 90%, will passthrough a 200 Tyler mesh screen, equivalent to 74 microns.

The small particle size filler can further comprise at least about 0.5%and up to about 25% of other additives like a puffing inhibitor such asiron oxide. Again, the additive should also be employed at a particlesize smaller than that conventionally used. For instance, when ironoxide is included, the average diameter of the iron oxide particlesshould be such that they are smaller than about 10 microns. Anotheradditional additive which can be employed is petroleum coke powder,having an average diameter such that they are smaller than about 10microns, added to fill porosity of the article and thus enable bettercontrol of the amount of pitch binder used. The small particle sizefiller should comprise at least about 30%, and as high as about 50% oreven 65% of the particulate fraction.

After the blend of particulate fraction, pitch binder, etc. is prepared,the body is formed (or shaped) by extrusion though a die or molded inconventional forming molds to form what is referred to as a green stock.The forming, whether through extrusion or molding, is conducted at atemperature close to the softening point of the pitch, usually about100° C. or higher. The die or mold can form the article in substantiallyfinal form and size, although machining of the finished article isusually needed, at the very least to provide structure such as threads.The size of the green stock can vary; for electrodes the diameter canvary between about 220 mm and 700 mm.

After extrusion, the green stock is heat treated by baking at atemperature of between about 700° C. and about 1100° C., more preferablybetween about 800° C. and about 1000° C., to carbonize the pitch binderto solid pitch coke, to give the article permanency of form, highmechanical strength, good thermal conductivity, and comparatively lowelectrical resistance, and thus form a carbonized stock. The green stockis baked in the relative absence of air to avoid oxidation. Bakingshould be carried out at a rate of about 1° C. to about 5° C. rise perhour to the final temperature. After baking, the carbonized stock may beimpregnated one or more times with coal tar or petroleum pitch, or othertypes of pitches or resins known in the industry, to deposit additionalcoke in any open pores of the stock. Each impregnation is then followedby an additional baking step.

After baking, the carbonized stock is then graphitized. Graphitizationis by heat treatment at a final temperature of between about 2500° C. toabout 3400° C. for a time sufficient to cause the carbon atoms in thecoke and pitch coke binder to transform from a poorly ordered state intothe crystalline structure of graphite. Advantageously, graphitization isperformed by maintaining the carbonized stock at a temperature of atleast about 2700° C., and more advantageously at a temperature ofbetween about 2700° C. and about 3200° C. At these high temperatures,elements other than carbon are volatilized and escape as vapors. Thetime required for maintenance at the graphitization temperature usingthe process of the present invention is no more than about 18 hours,indeed, no more than about 12 hours. Preferably, graphitization is forabout 1.5 to about 8 hours. Once graphitization is completed, thefinished article can be cut to size and then machined or otherwiseformed into its final configuration.

In order to provide a pin configured to form an electrode joint havingimproved stability in the furnace, the male tang, and, most preferably,both male tangs, of the pin (and, by extension, the female socket of theelectrodes into which the pin is to be threaded) must be dimensionedsuch that the tang will provide the required strength in use, even wherethe pin is formed of the same graphite material from which theelectrodes are formed in order to match thermal properties. In order todo so, a balancing must be accomplished. More particularly, it is nowbeen discovered that the ratio of the length of the male tang to thediameter of the electrode into which it is to be threaded (referred toherein as the tang factor) is important in optimizing the performance ofthe joint. More specifically, a tang factor of at least about 0.60 isbelieved to be important in creating an electrode joint having improvedstability and commercially acceptable performance.

The interaction of other joint characteristics can also help optimizethe electrode joint. For instance, a ratio (referred to herein as thetang diameter factor) of a factor defined by the ratio of the diameterof the male tang of the pin at its base to the male tang length can beused to provide even further enhancements to the joint. The tangdiameter factor should be no greater than 2.5 times the tang factor foran especially effective joint with a tang factor of about 0.60. Indeed,the tang diameter factor should most preferably vary with the tangfactor, such that when a joint with a tang factor higher than 0.60 isproduced, the tang diameter factor of the joint should be lower than 2.5times the stub factor. More specifically, for every 0.01 higher than0.60 that the tang factor of a joint is, the maximum tang diameterfactor should be about 0.016 lower. As an example, when a joint having atang factor of 0.85 is produced, the tang diameter factor of the maletang of the joint should be lower than about 1.28 times the tang factorof the joint.

Another joint characteristic that can come into play in designing aneffective graphite electrode joint is referred to herein as the taperfactor, which is defined as the ratio of the taper (expressed indegrees, and illustrated in FIG. 1 as the angles designated a) of themale tang to the tang factor. The taper factor for an effective maletang should be at least about 15, where the tang factor is 0.85, andshould also vary as pins with different tang factors are produced. Forinstance, for every 0.01 lower than 0.85 that the tang factor of a pinis, the minimum taper factor should be about 1.25 higher. As an example,when a pin having a tang factor of 0.60 is produced, the taper factor ofthe male tang of the pin should be at least about 45.

When employing the tang factor of at least about 0.60, and/or the tangdiameter factor or taper factor of the pin as described above, a jointis produced that can achieve commercial acceptability, at least in termsof joint strength and stability. A typical graphite electrode jointproduced in accordance with the invention is illustrated in FIGS. 1–3and denoted 10. Joint 10 comprises a pin 20, a first electrode 100 and asecond electrode 110, pin 20 having a first male tang 20 a and a secondmale tang 20 b. First electrode 100 and second electrode 110 each have afemale socket 30. As illustrated, male tangs 20 a and 20 b, and femalesockets 30 cooperate to form joint 10 and thus connect first electrode100 and second electrode 110 into a column. With proper dimensioning ofmale tangs 20 a and 20 b (and corresponding dimensioning of femalesockets 30), an improved joint 10 is provided.

The disclosures of all cited patents and publications referred to inthis application are incorporated herein by reference.

The above description is intended to enable the person skilled in theart to practice the invention. It is not intended to detail all of thepossible variations and modifications that will become apparent to theskilled worker upon reading the description. It is intended, however,that all such modifications and variations be included within the scopeof the invention that is defined by the following claims. The claims areintended to cover the indicated elements and steps in any arrangement orsequence that is effective to meet the objectives intended for theinvention, unless the context specifically indicates the contrary.

1. A process for preparing a pin for use in forming a graphite electrodejoint, the process comprising (a) mixing coke and a pitch binder, toform a stock blend; (b) extruding the stock blend to form a green stock;(c) baking the green stock to form a carbonized stock; (d) graphitizingthe carbonized stock by maintaining the carbonized stock at atemperature of at least about 2500° C. to form a graphitized stock; (e)machining the graphitized stock so as to form a pin having at least onemale tang having a ratio of male tang length to diameter of theelectrode into which it is to be threaded of at least about 0.60,wherein the ratio of the diameter of the male tang at its base to maletang length is no greater than about 2.5 times the ratio of male tanglength to electrode diameter.
 2. The process of claim 1, wherein theratio of the diameter of the male tang at its base to male tang lengthvaries with the ratio of male tang length to electrode diameter suchthat for every 0.01 higher than 0.60 the ratio of male tang length toelectrode diameter is, the ratio of the diameter of the male tang at itsbase to the ratio of male tang length to electrode diameter is about0.016 lower.
 3. The process of claim 1, wherein the male tang has ataper and further wherein for a pin having a ratio of male tang lengthto electrode diameter of 0.85 or lower, the ratio of the taper of themale tang to the ratio of male tang length to electrode diameter is atleast about
 15. 4. The process of claim 3, wherein the ratio of thetaper of the male tang to the ratio of male tang length to electrodediameter varies with the ratio of male tang length to electrode diametersuch that for every 0.01 lower than 0.85 the ratio of male tang lengthto electrode diameter is, the ratio of the taper of the male tang to theratio of male tang length to electrode diameter is about 1.25 higher.